The present invention relates to a filter wherein double-mode resonance is caused in the resonator with a dielectric resonator being accommodated within a cut-off waveguide having a given axial length.
An elliptic function-type filter is known, wherein a plurality of cavity resonators are disposed in the longitudinal columns, coupling slots are provided in the faces of the cavity resonators for crossing the propagation-direction axis of the electromagnetic-field energy, the double-mode resonance is caused with a dielectric resonator element being accommodated within each cavity resonator, and each stage is coupled through the coupling slot. See Japanese Patent Application (unexamined) Publication Tokkai-sho No. 57-194603 which is equivalent to U.S. Pat. 4,489,293. However, this known construction uses a compound type of resonator composed of a cavity resonator and a dielectric resonator element disposed within the cavity resonator, which has the difficulty in production that a bulkhead having the coupling slot has to be provided as the boundary face between the adjacent cavity resonators for the coupling operation among each resonators. Also, the conductor loss is caused because of the existence of the coupling slot, thus resulting in more insertion loss.